Traveling wire grade control apparatus for slipform machines

ABSTRACT

A slipform paving machine having a vertically adjustable screed which is automatically controlled by a traveling wire apparatus to maintain grade by directly sensing the height of the screed relative to the surface to be paved. The traveling wire apparatus includes a pair of rigid bars secured to the sides of the screed to project forwardly and rearwardly from the machine, and sensing units comprising frames, taut wires, and switches are pivotally attached to each of the ends of the bars. The frames move relative to the bars any time the profile of the surface changes and the wires actuate the switches which control the height of the screed whenever relative movement between the frames and bars occurs.

United States Patent [191 Tate TRAVELING WIRE GRADE CONTROL APPARATUS FOR SLIPFORM MACHINES [75] Inventor: William H. Tate, Mira Loma, Calif.

[73] Assignee: Owl Slipform Concrete C0.,

Compton, Calif.

[22] Filed: Sept. 28, 1972 [21] Appl. No.: 292,997

[52] US. Cl. 404/84 [51] Int. Cl. E01c 19/00 [58] Field of Search 404/84, 105, 108, 118

[56] References Cited UNITED STATES PATENTS 2,864,452 12/1958 Guntert 404/84 X 3,272,099 9/1966 Drake 404/84 3,618,484 ll/l97l Long 404/84 3,323,427 6/1967 Schrimper.... 404/84 3,636,833 l/l972 Lowen 404/84 3,540,360 1l/l970 Snow 404/84 3,334,560 8/1967 Long 404/84 2,962,979 12/1960 McCormick 404/84 X 2,491,275 12/1949 Millikin 404/84 Primary Examiner-Nile C. Byers, Jr. Attorney, Agent, or Firm-Fulwider, Patton, Rieber, Lee & Utecht [57] ABSTRACT and sensing units comprising frames, taut wires, and

switchesare pivotally attached to each of the ends of the bars. The frames move relative to the bars any time the profile of the surface changes and the wires actuate the switches which control the height of the screed whenever relative movement between the frames and bars occurs.

20 Claims, 7 Drawing Figures TRAVELING WIRE GRADE CONTROL APPARATUS FOR SLIPFORM MACHINES BACKGROUND OF' THE INVENTION This invention relates to slipform paving machines, and has particular reference to a new and improved apparatus for automatically controlling the grade of a slab laid by such machines.

Numerous attempts have been made at devising devices for automatically controlling slipform machines to lay a slab, such as for a road or a canal, which will provide a grade reference for following a prescribed profile to meet grade requirements as closely as possible. One of the most successful of such attempts has been the use of sensors attached to the frame of the machine and which take the grade from a pair of wire lines stretched taut over steel arms on grade stakes along each of the sides of the bed to be paved.

The sensors, one on each of the four corners of the frame, straddle the wires and elevate or lower the slipform pan or screed in response to vertical movement of the sensors relative to the stationary wires. In a typical case, if the machine encounters a rise in the bed, the frame, which usually is propelled over the'bed by endless tracks, will rise relative to the wire and the sensors will correspondingly move and signal the screed to descend, there by to smooth over the rise and maintain the grade.

While the use of automatic grade reference devices of the foregoing character has increased the accuracy of the grade of a slab laid by a slipform machine, the cost of surveying and setting the wire lines on each side of the bed is quite expensive. Further, the use of fixed wire lines poses a problem of accessibility for getting paving material to the front of the slipform machine, this typically being done by continuously dumping truck loads of paving material in front of the machine. In order to provide access for the trucks, it is necessary to provide gaps in the wire line ahead of the machine, which gaps must constantly be resurveyed and the wire lines reset as the paving operation progresses.

To solve some of the problems inherent in the use of fixed wire lines, attempts have also been made at providing traveling wire lines supported by the frame of the machine. One example of such a device is that shown and described in U.S. Pat. No. 3,323,427 which employs an elongated ski pivotedly attached to the side of the machine frame for sliding along the bed, and which carries a taut wire line. Relative movement between the ski and the machine is detected by a grade sensor supported by the frame of the machine and coupled with the screed control, and functions to raise or lower the screed in response to sensed displacement of the frame of the machine.

Although traveling wire devices such as that in U.S. Pat. No. 3,323,427 solve many of the problems inherent in fixed-wire devices, it has been found that the accuracy of the grade of the slab produced by machines employing traveling-wire devices has been somewhat less than satisfactory. The present invention, on the other hand, provides a traveling-wire grade control apparatus which controls the height of the screed in such a manner that the grade of the slab is maintained to within very small and acceptable tolerances.

trol apparatus which senses the height of the screed of 4 a slipform machine relative to the bed being paved, and automatically and continuouslycorrects the screed to fill in depressions and smooth over rises. Grade control is accomplished by attaching traveling-wire linesensing units directly to the screed thereby to sense directly the height of the screed relative to the bed.

More specifically, the grade control apparatus includes two rigid bars secured one to each side of the screed and extending forwardly and rearwardly from the slipform machine. Attached to each of the ends of the bars is a traveling-wire line-sensing unit which includes a taut wire and sensor for independently controlling the height of one corner of the screed.

Each of the wires is secured at one of its ends to the bar and at its other end to a frame which is supported for movement over the bed and pivotally coupled to the end of the bar. Each sensor is adjustably attached to one end of the associated bar and straddles the associated wire to sense relative movement between the frame and the bar anytimethe frame encounters a rise or fall in the bed.

With the bars attached'directly to the screed, the height of the screed above the bed is sensed directly, independent of movement of the frame of the machine. Further, the sensing units can be offset laterally of the machine to insure that the grade profile followed is that of the desired surface, either the bed actually being ,paved, or an adjacent surface such as a previously laid DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a slipform paving machine employing a grade control-apparatus in accordance with the inventionfor laying a slab on a prepared bed; g

FIG. 2 is a fragmentary side elevation of the right side of the slipform machine and grade control apparatus shown in FIG. 1 without the track motor and propulsion apparatus, and having portions removed for compact illustration; 7 I

FIG. 2a is a fragmentary sectional view taken substantially along line 2a2a of FIG. 1;

FIG. 3 is an enlarged isolated view of the grade control apparatus for the right side of the slipform machine of FIG. 1 and shown removed therefrom;

FIG. 4 is an enlarged, fragmentary sectional view taken substantially along line 4-4 of FIG. 3;

FIG. 5 is an enlarged, fragmentary sectional view taken substantially along line 55 of FIG. 5; and

FIG. 6 is a diagrammatic representation of the operation of a portion of the grade control apparatus of the invention when encountering a rise in the bed being paved, the rise being exaggerated for purposes of illustration.

DETAILED DESCRIPTION As shown in the exemplary drawings, the present invention is embodied in a grade control apparatus for use with a slipform paving machine 12 to control the grade of a slab 14 of paving material laid by the machine on a prepared base or bed 16. As shown in FIG. 1, the slipform machine 12 is of basically conventional design, and includes an elongated platform-like carriage 18 spanning the width of the bed 16 to be paved and supporting a slipform pan or screed 20 (see FIG. 2) which forms the slab by compacting and smoothing paving material into a dense, uniform mass of preselected thickness and width.

The machine 12 is driven over the bed by a pair of motor-powered endless tracks 22 coupled to the ends of the carriage 18 through fluid operated cylinders 24, herein hydraulic cylinders, which are vertically disposed on mounts 26 attached to the frames of the tracks. The cylinders 24, of which there are four in all, are coupled to the carriage 18 at each corner thereof, and are independently controlled to raise or lower the respective corners of the carriage relative to the tracks 22 in response to the admission or discharge of hydraulic fluid from the individual cylinders.

' As can be best seen in FIGS. 1 and2a, each cylinder 24 includes a piston and rod 28 projecting upwardly from the mount 26 and disposed in a vcylindrical housing 30 attached to one end portion of a beam 32 to which the end of the carriage 18 is bolted through an upstanding bracket 34. Each of the front cylinders 24 is rigidly connected to the beam 32, while the rear cylinders are connected to the beams through pivotal attachments 33 which allow the beams to pivot and move in a vertical plane. The carriage 18 is raised by actuating a control valve in the fluid conduit to each cylinder 24 to admit fluid to the upper ends of the cylindrical housings 30, thereby forcing the beams 32 to rise relative to the brackets 26 and tracks 22.

The screed 20 extends across and forms the underside of the carriage l8 and comprises a rigid pan having a flat bottom 36 with upturned sides 38 and an upwardly and rearwardly inclined front 40 extending across the front of the carriage. To form the slab 14, paving material, herein wet concrete, is dumped, usually by the truck load, immediately in front of the slipform machine 12 and is forced under the screed 20 by the forward motion of the machine. The inclined front 40 of the screed 20 is gradually merged into the flat bottom 36 by a rearwardly and downwardly curved section 42 which functions as a constricting passageway to compact the concrete as it moves under the bottom the screed.

To prevent paving material from escaping laterally of the machine 12 as it advances, and to form flat sidewalls on the slab 14, vertical side forms 44 are secured to the ends of the carriage 18 laterally inwardly of the tracks 22. Referring primarily to FIGS. 2 and 2a, it can be seen that the side forms 44 include bottom strips 46 which are slidably attached by bolts through elongated slots 48 to the main upper portions of the forms, and slide along the bed 16 to keep the paving material under the screed as the slab 14 is formed. During raising or lowering of the screed 20, the strips 46 slide relative to the upper portions of the forms 44 to remain engaged with the bed 16.

In order to obtain maximum density of the concrete in the slab 14, a tubular vibrator 50 is attached to the front 40 of the screed 20 and agitates the wet concrete into a substantially liquid state prior to being compacted and formed by the screed. Agitation forces entrained air out of the wet concrete, thereby preventing air bubbles and voids from being formed in the slab as the concrete is compacted into a dense mass under the screed 20.

In accordance with the present invention, the grade control apparatus is self-contained and automatically and continuously regulates the height of the screed 20 relative to the bed 16 being paved, to insure that the surface of the slab 14 along its length is uniform and smooth within very small tolerances and to produce a slab of constant thickness over the bed. Further, the grade control apparatus performs this function in a highly reliable and effective manner by directly sensing the height of the screed 20 relative to thebed l6 and continuously correcting the screed height to compensate for variations in the contour of the bed during the paving operation in order to fill-in depressions and smooth-over rises in the bed. I

Toward the foregoing ends, the grade control apparatus includesfour sensor units 52 attached to the ends of two elongated and rigid barsv54 which are secured to the sides of the screed 20 and project both forwardly and rearwardly from the slipform machine 12, one end portion of one of the bars thus projecting outwardly away from the screed from each corner thereof. Each sensor unit 52 comprises an arm 55 connected to the end portion of the bar 54 for pivotal movement relative to the bar in a vertical plane, and is supported on the bed 16 for movement thereon by a traveller 53 which moves vertically with vertical changes in the grade of the bed. Means 56 are provided for detecting relative movement between each of the arms 55 and the adjacent barend, and for controlling each of the four cylinders 24 individually to adjust the height of the associated corner of the screed 20 in response to sensed relative movement between the associated bar and arm.

As shown in FIGS. 2, 2a, and 3, each of the bars 54 is rigidly attached to the side 38 of the screed 20 at two points, each such attachment herein comprising a section of an I-beam 58 welded to the screed and bolted to upstanding cylindrical pins 60 welded to the top of the bar. With this arrangement, any movement of the screed 20 will be transmittedto the bars 54 and result in movement of each bar in unison with the screed. Herein, each bar 54 is formed of three-sections, one section 62 of the bar extending forwardly and one section 64 extending rearwardly of the slipform machine 12, and each of these sections being releasably secured to a central section 66 by hinges 68 and bolted flanges 70. The hinges 68 allow the forward and rear sections 62 and 64 to be folded upwardly in the direction of the arrows of FIGS. 2 and 3 thereby to reduce the length of the bar 54 for transporting of the slipform machine 12 from one location to another between jobs.

Referring primarily to FIGS. 1 and 3, the forward sections 62 of each of the bars 54 include horizontally disposed L-shaped mounts 72 at the ends, which allow the forward sensor units 52 to be offset laterally relative to the bars. Each mount'72 is attached with one leg 74 of the L projecting through a collar 76 integral with the front of the bar 54, and a forwardly projecting leg 78 to which the arm 55 is attached. Each mount 72 can be adjusted to control the lateral offset of the arm 55 by first releasing several set screws 80 which extend through the sides of the collar 76 and engage the lateral leg 74 of the mount, and then sliding the mount through the collar to the desired position.

Although the rear sensor units 53 are attached directly to the rear sections 64 of the bars 54, it should be noted, as will be more fully explained hereinafter, that mounts similar to those shown on the front ends of the bars can be used if desired. The arms 55 of the forward and rear units 52 are similar in structure, the front arms being attached to the extensions 72, and the rear arms being attached directly to the rear sections 64 of the bars 54, as noted above.

Referring to FIG. 3, and considering the structure of each sensor unit 52 in relation to the forward unit shown therein, the arm 55 comprises a frame formed of a pair of spaced parallel rails 82 projecting forwardly from each side of the forward leg 78 of the mount 72, with the rear end portions of the rails interconnected by a shaft 84 extending through an opening in the mount and pivotally mounted therein for movement about a horizontal axis. The forward ends of the rails 82 are connected by an elongated rectangular'plate 85 extending across and welded to the front ends of the rails. A collar 86 is secured to the rearward side of the plate around a post 87 upstanding from a pair of tandem wheels 88 forming the trailer 53 which rides on the bed 16. Herein, the wheels 88 are rotatably mounted between spaced plates 89 pivotablyattached at their centers to opposite sides of the post 87 adjacent the lower end.

As the machine 12 moves over the bed 16, the arm 55 will pivot relative to the end of the bar 54 anytime the wheels 88 encounter a change in the contour of the bed. To sense relative movement between the arm 55 and the bar 54, a wire 90 is stretched taut between the upper portion of the post 87 of the frame and a sensor mount, herein a post 95, secured to the rear portion of the forward leg 78 of the mount 72 and upstanding therefrom. The wire 90 is attached to each post 87 and 95 by eye bolts 91 and an extension spring 93 which maintains tension in thewire. Between the posts 87 and 95, and straddling the wire 90, are two vertically spaced sensors 94 mounted on a housing 96 secured to the front portion of the forward leg 78 of the mount 72. Any movement of the. arm 55 relative to the bar 54 will produce a vertical displacement of the wire 90 with the result that one of the sensors 94 willbe actuated and signal the control valve of a cylinder 24'to raise or lower the screed 20.

As can best be seen in FIGS. 4 and 5, the sensors 94' comprise finger-like actuators 98 pivotally attached at one end to the housing 96 and projecting across the wire 90, one finger projecting over the top of the wire and the other under the wire. The fingers 98 are protected by a forked cover 99 attached to the side of the housing 96, the cover allowing only one edge portion of each finger to engage the wire 90 and serving to limit deflection of the fingers by the wire. Between the wire 90 and each attachment of the fingers 98 to the housing 96, are a pair of switches 100 mounted in the housing. The switches 100 herein are conventional off-on devices operated by plungers 102 which, inturn, are deflected by movement of the fingers 98 in response to vertical movement of the wire 90.

Eachswitch 100 is secured to the side of the housing 96 with its plunger 102 in engagement with a laterally projecting tab 104 integrally formed on the edge of the finger 98 remote from the wire 90. As viewed in FIGS.

4 and 5, the upper switch 100 has its plunger 102 projecting downwardly into engagement with the finger 98 which straddles the top of the wire 90, and the lower switch is mounted so that its plunger projects upwardly into engagement with the finger straddling the underside of the wire. In this way, deflection of the wire will cause one of the fingers 98 to-pivot, which in turn will depress the associated plunger 102 to turn its switch from the of to the on position.

The housing 96 is generally box-shaped and is adjustably mounted on the mount 72 to project thereabove. In this instance, the housing 96 has a threaded sleeve 106 which encircles a screw 108 extending between the upper and lower legs 110 of a C-shaped bracket 112 welded to the mount 72 and upstanding therefrom. Vertical adjustment of the housing 96 is accomplished by rotating the screw 108 through a hand wheel 114 attached to the top of the screw above the upper leg 110 of the bracket 112 to cause the threaded sleeve 106 to travel along the screw.

Each of the pairs of switches 100 is interconnected with the control valve for the fluid cylinder 14 associated with the corner of the machine 12 closest to the particular sensor unit 52, to either admit fluid to, or bleed fluid from, that cylinder. For example, the right front sensor unit 52 operates the right front cylinder 24, the left rear sensor unit operates the left'rear cylinder, and so forth, and in each case the upper of the two switches 100 controls the bleed of fluid from one cylinder and the lower switch controls the admission of fluid.

Since each of the four cylinders 24 controlling the height of the screed 20 is independently controlled by the grade control apparatus, very accurate and precise positioning of the screed is maintained at all times. Further, since the position of the screed 20 relative to the bed 16. is'directly sensed to control the cylinders 24, the grade of the slab 14 will correspond, within very small tolerances, to the true grade of the bed notwithstanding the presence of irregularities therein.

To commence operation of the machine 12, the height of the screed 20 above the bed 16 is initially determined by the thickness of the slab 14 to be laid. The screed 20 is then raised to the selected height above the bed 16 bymanual control of the cylinders 24, typically by an operator standingion a platform on the carriage 18 who monitors a control panel which includes manual overrrides .for the automatic controls of the machine 12. After the screed 20 has been raised, the sensing unit 52 must be adjusted to maintain the preselected height of the screed 20 above the bed 16. This is accomplished by adjusting the positions of the posts 87 of the arms 54 until the rails 82 of the arms and the wires 90 are parallel with the bars 52. Each of the housings 96'is then adjusted by rotation of the screw 108 until the wire 90 is midway between the fingers 98 and both switches 100' are in the"off position.

Normally, when layingaslab 14 on a prepared or gradedbed 16, the bed-is typically formed to be only a foot or so wider than the slab to be laid. To insure that the grade of the slab 14 is controlled by the grade of the actual bed to be paved, as opposed to the more coarse and rough surfaces along the sides of the bed, the positions of the forward mounts 72 should be adjusted to allow the wheels 88 of the arms 55 to travel on the bed itself; This can be accomplished by releasing theset screws 80 and laterally adjusting the positions of the legs 74 of the mounts 72 relative to the collars 76.

In some instances it may be necessary to lay a slab 14 along the side of a previously laid slab. In such a case, the new slab, where it joins the first slab, should match the contour of the first slab as closely as possible. To

achieve this, mounts similar to the mounts 72 shownherein on the fronts of the bars 52 also can be used on the rear sections 64. The positions of the wheels 88 of the front and rear arms 55 adjacent the first slab can then be adjusted laterally by reversing the direction of off-set shown in FIG. 3 to ride on the surface of the first slab thereby to control the height of the screed 20 by the contour of the first slab.

When fully adjusted, the slipform machine 12 can commence operation and any depression or rise in the bed 16 will result in relative movement of the arms 55 and bars 52 and, consequently, movement of the wire 90 and actuation of one of the switches 100. F011 example, as shown in the diagram of FIG. 6, when the slipformmachine 12 approaches a rise in the bed 16, the wheels 88 of the forward arms 55 will cause the rails 82 to pivot upwardly relative to the bars 52, thus displacing the wires 90, causing the upper fingers 98 to move and depress the plungers 102 of the upper switches 100, and thereby signaling the associated cylinders 24 to lower the screed 20. As the tracks 22 of the machine 12. ride over the rise, the entire machine will rise. The lowering of the screed 20 produces a reduction in the thickness of the concrete laid over the rise in the bed 16, thereby maintaining the grade of the slab 14 across the rise.

Since the tandem wheels 88 of the arms 55 are pivotally mounted to the frames, small holes or rises, such as pot holes or protruding rocks, will have little affect on the screed 20, as each wheel will simply drop in the hole or ride over the rock and immediately return. Thus, due to the inherent time lag between actuation of the control valve for a given cylinder 24 and actual movement of the screed 20, transient conditions of short duration, such as those caused by pot holes or rocks, will not change the grade of the slab l4 laid by the machine 12.

From the foregoing, it should be apparent that the grade control apparatus of the invention functions quickly and reliably to control the height of the screed 20 relative to the bed 16, thereby to produce a slab 14 of substantially uniform thickness along its length. Since the grade control apparatus is self-contained, the

amount of labor and time heretofore necessary to position and maintain a grade control line have been eliminated.

Further, since the grade control apparatus is tied directly to the screed 20 and independent sensor units 52 are used to sense and control each corner of the screed, very precise and accurate screed position can be maintained. In actual tests, it has been found that the apparatus will enable a slipform machine to lay a slab which is true to grade to below seven tenths inch per mile, a tolerance heretofore thought attainable only with precisely surveyed and set fixed-wire line devices.

While a particular form of the invention has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention.

1 claim:

1. In combination with a slipform paving machine having a screed for spreading paving material over a surface, and means for adjusting the height of the screed relative to the surface, a grade control apparatus comprising:

a pair of rigid bars secured to the sides of the screed and each extending both forwardly and rearwardly from the slipform machine;

four arms, one attached to each end of each of said bars and supported for movement over the surface to be paved, said arms being pivotally attached to said bars for vertical movement;

and four independent sensor means, one coupled with each arm, per sensing relative vertical movement between said bars and said arms, and for actuating the adjusting means to control the height of the screed in response to said sensed relative movement.

2. The combination as defined in claim 1 in which each of said independent sensor means comprise:

a taut wire connected at one end to said arm and at the other to said bar;

and a pair of switch actuators adjacent said wire and attached to said bar, said actuators being connected to the screed adjusting means and selectively actuated by said wire in response to vertical movement of said arm relative to said bar.

3. The combination as defined in claim 2 wherein said switch actuators are operated by a pair of fingers straddling'said wire, and the screed adjusting means comprise fluid cylinders-connecting each corner of the screed with the machine, said switch actuators being connected to said adjusting means through control valves which selectively admit to or discharge fluid from said cylinders. 1

4. The combination as defined in claim 2 wherein mounts are included for laterally offsetting said arms from the ends of said bars.

5. The combination as defined in claim 4 in which said' laterally offsetting mounts comprise L-shaped members horizontally disposed and adjustably secured to the ends of said bars.

6. The combination as defined in claim 1 in which each of said arms is supported for movement over the surface by a traveler comprising a pair of tandem wheels supporting one end of said arms with the other end of said arm pivotally connected to said bar.

7. In combination 'with a slipform paving machine having a vertically adjustable screed for spreading paving material over a surface, a grade control apparatus comprising: p v

a pair of rigid bars secured to the sides of the screed and each extending forwardly from the slipform machine;

a pair longitudinally disposed arms each having one end pivotally attached to the forward end portion of one of saidvbars for vertical movement relative to said bar;

means for supporting the other ends of each of said arms on the surface for movement thereon;

means for sensing relative vertical movement between said bars and said arms; and

means coupled withsaid sensing means for vertically each of said means for supporting said other ends of said arms include a traveler moveable on said surface and a post projecting vertically above said traveler;

each of said sensing means include a longitudinally disposed taut wire having one end attached to said bar and the other end attached to said vertical post; and

said means coupled with said sensing means include a pair of switch actuators, one positioned above and the other positioned below said wire and selectively actuated by movement of said wire.

9. The combination as defined in claim 8 in which one of said switch actuators is actuated when said wire is moved upwardly relative to said bar, and the other of said switch actuators is actuated when said wire is moved downwardly relative to said bar, and said wire is moved in response to said traveler moving onsaid surface and being vertically displaced due to a change in profile of said surface.

10. The combination as defined in claim 9 wherein said one switch actuator operates to lower the height of said screed and said other switch actuator operates to raise said screed whereby the height of said screed is lowered when the profile of the surface increases and is raised when the profile decreases.

11. In combination withaslipform paving machine having a screed for spreading paving material over a surface and means for adjusting the vertical height of the screed above the surface, a traveling wire grade control apparatus comprising:

a rigid bar secured to the side of the screed and projecting forwardly and rearwardly therefrom; and sensing units attached adjacent each end of said bar, each of said sensing units including a longitudinally disposed arm having one end pivotally attached to said bar for up and down movement, with the other end supported by the surface to be paved and moveable thereover, a taut wire attached at one end to said arm and at the other end to said bar, and switch actuator means attached to said arm and actuated by movement of said wire in response to relative movement between said arm and said bar for controlling the vertical position of the screed. 12. In combination with a slipform paving machine having a screed for spreading paving material over a surface and means for adjusting the vertical height of the screed above the surface, a traveling wire grade control apparatus comprising:

a rigid bar secured to the side of the screed and projecting forwardly and rearwardly therefrom;

sensing units attached adjacent each end of said bar, each of said sensing units including a longitudinally disposed arm having one end pivotally attached to said bar for up and down movement, with the other end supported by the surface to be paved and moveable thereover, a taut wire attached at one end to said arm and at the other end to said bar, and switch actuator means attached to said arm and actuated by movement of said wire in response to relative movement between said arm and said bar for controlling the vertical position of the screed;

and means for laterally offsetting the position of at least one of said arms relative to said bar.

13. In combination with a slipform paving machine having a screed for spreading paving material over a surface and means for adjusting the vertical height of the screed above the surface, a traveling wire grade control apparatus comprising:

a rigid bar secured to the side of the screed and projecting forwardly and rearwardly therefrom;

and sensing units attached adjacent each end of said bar, each of said sensing units including a longitudinally disposed arm having one end pivotally attached to said bar for up and down movement, with the other end supported by the surface to be paved and moveable thereover by a pair of tandem wheels rollable over said surface and attached to said frame for pivotal movement in a vertical plane, a taut wire attached at one end to said arm and at the other end to said bar, and switch actuator means attached to said arm and actuated by movement of said wire in response to relative movement between said arm and said bar for controlling the vertical position of the screed, said switch actuator means including a pair of spaced fingers'pivotally attached at one end to said bar and straddling said wire, said fingers being coupled with a pair of switches which operate to control actuation of the screed adjusting means.

14. The combination as defined in claim 13 wherein said fingers are attached to a housing adjacent one side of said wire and between the ends thereof, said housing being adjustable vertically relative to said bar.

15. In combination with a slipform paving machine having a screed for spreading paving material over a surface, and selectively operable means for adjusting the height of the screed relative to the'surface, a grade control apparatus having:

at least one elongated, rigid bar secured to said screed and extending forwardly therefrom to overlie the surface in advance of the machine; and

a sensing unit mounted on the forward end portion of said bar to travel along the surface, and comprising;

a sensor mount supported on said bar;

an arm pivotally supported on said forward end por-- vertically from said position in response to vertical I movement of said arm; and means actuated by vertical movement of said member in either direction from said normal operating position, and operable to operate said selectively operable means to adjust the height of said screed. 16. The combination as defined in claim 15 in which said means actuated by vertical movement comprise a pair of vertically spaced fingers straddling said member, a pair of actuators which control operates said selectively operable means, and coupled one with each of said fingers.

17. The combination as defined in claim 15 in which said taut member comprises a wire stretched between said mount and said arm, and said means actuated by said vertical movement comprise switch actuators coupled with said selectively operable means and supported by said bar, said actuators having spacedoperators straddling said wire to sense vertical movement thereof.

18. In combination with a slipform paving machine having a screed for spreading paving material over a surface, and selectively operable means for adjusting the height of the screed relative to the surface, a grade control apparatus having:

at least one elongated, rigid bar secured to said screed and extending forwardly therefrom to overan arm pivotally supported on said forward end portion of said bar for vertical movement relative to said sensor mount, and having means thereon for traveling on said surface and moving said arm vertically in response to vertical changes in the contour I 12 of said surface, said means for traveling including a travelerhaving a pair of tandem wheels;

a taut member extending between said mount and arm in a normal operating position and moveable vertically from said position in response to vertical movement of said arm;

and means actuated by vertical movement of said member in either direction from said normal operating position, and operable to operate said selectively operable means to adjust the height of said screed.

19. The combination as defined in claim 18 in which said taut member comprises a wire stretched between said adjustable mount and said arm, and said means actuated by said vertical movement comprise switch actuators adjacent said wire and supported by said bar, said actuators being connected to said selectively operable means to control said selectivelyoperable means to adjust the height of said screed in response to vertical displacement of said wire from said normal position.

20. The combination as defined in claim 19 in which said arm comprises a pair of spaced longitudinally projecting rails and said sensormount comprises a post upstanding from said adjustable mount. 

1. In combination with a slipform paving machine having a screed for spreading paving material over a surface, and means for adjusting the height of the screed relative to the surface, a grade control apparatus comprising: a pair of rigid bars secured to the sides of the screed and each extending both forwardly and rearwardly from the slipform machine; four arms, one attached to each end of each of said bars and supported for movement over the surface to be paved, said arms being pivotally attached to said bars for vertical movement; and four independent sensor means, one coupled with each arm, per sensing relative vertical movement between said bars and said arms, and for actuating the adjusting means to control the height of the screed in response to said sensed relative movement.
 2. The combination as defined in claim 1 in which each of said independent sensor means comprise: a taut wire connected at one end to said arm and at the other to said bar; and a pair of switch actuators adjacent said wire and attached to said bar, said actuators being connected to the screed adjusting means and selectively actuated by said wire in response to vertical movement of said arm relative to said bar.
 3. The combination as defined in claim 2 wherein said switch actuators are operated by a pair of fingers straddling said wire, and the screed adjusting means comprise fluid cylinders connecting each corner of the screed with the machine, said switch actuators being connected to said adjusting means through control valves which selectively admit to or discharge fluid from said cylinders.
 4. The combination as defined in claim 2 wherein mounts are included for laterally offsetting said arms from the ends of said bars.
 5. The combination as defined in claim 4 in which said laterally offsetting mounts comprise L-shaped members horizontally disposed and adjustably secured to the ends of said bars.
 6. The combination as defined in claim 1 in which each of said arms is supported for movement over the surface by a traveler comprising a pair of tandem wheels supporting one end of said arms with the other end of said arm pivotally connected to said bar.
 7. In combination with a slipform paving machine having a vertically adjustable screed for spreading paving material over a surface, a grade control apparatus comprising: a pair of rigid bars secured to the sides of the screed and each extending forwardly from the slipform machine; a pair longitudinally disposed arms each having one end pivotally attached to the forward end portion of one of said bars for vertical movement relative to said bar; means for supporting the other ends of each of said arms on the surface for movement thereon; means for sensing relative vertical movement between said bars and said arms; and means coupled with said sensing meanS for vertically adjusting said screed in response to said sensed relative movement.
 8. The combination as defined in claim 7 in which: each of said means for supporting said other ends of said arms include a traveler moveable on said surface and a post projecting vertically above said traveler; each of said sensing means include a longitudinally disposed taut wire having one end attached to said bar and the other end attached to said vertical post; and said means coupled with said sensing means include a pair of switch actuators, one positioned above and the other positioned below said wire and selectively actuated by movement of said wire.
 9. The combination as defined in claim 8 in which one of said switch actuators is actuated when said wire is moved upwardly relative to said bar, and the other of said switch actuators is actuated when said wire is moved downwardly relative to said bar, and said wire is moved in response to said traveler moving on said surface and being vertically displaced due to a change in profile of said surface.
 10. The combination as defined in claim 9 wherein said one switch actuator operates to lower the height of said screed and said other switch actuator operates to raise said screed whereby the height of said screed is lowered when the profile of the surface increases and is raised when the profile decreases.
 11. In combination with a slipform paving machine having a screed for spreading paving material over a surface and means for adjusting the vertical height of the screed above the surface, a traveling wire grade control apparatus comprising: a rigid bar secured to the side of the screed and projecting forwardly and rearwardly therefrom; and sensing units attached adjacent each end of said bar, each of said sensing units including a longitudinally disposed arm having one end pivotally attached to said bar for up and down movement, with the other end supported by the surface to be paved and moveable thereover, a taut wire attached at one end to said arm and at the other end to said bar, and switch actuator means attached to said arm and actuated by movement of said wire in response to relative movement between said arm and said bar for controlling the vertical position of the screed.
 12. In combination with a slipform paving machine having a screed for spreading paving material over a surface and means for adjusting the vertical height of the screed above the surface, a traveling wire grade control apparatus comprising: a rigid bar secured to the side of the screed and projecting forwardly and rearwardly therefrom; sensing units attached adjacent each end of said bar, each of said sensing units including a longitudinally disposed arm having one end pivotally attached to said bar for up and down movement, with the other end supported by the surface to be paved and moveable thereover, a taut wire attached at one end to said arm and at the other end to said bar, and switch actuator means attached to said arm and actuated by movement of said wire in response to relative movement between said arm and said bar for controlling the vertical position of the screed; and means for laterally offsetting the position of at least one of said arms relative to said bar.
 13. In combination with a slipform paving machine having a screed for spreading paving material over a surface and means for adjusting the vertical height of the screed above the surface, a traveling wire grade control apparatus comprising: a rigid bar secured to the side of the screed and projecting forwardly and rearwardly therefrom; and sensing units attached adjacent each end of said bar, each of said sensing units including a longitudinally disposed arm having one end pivotally attached to said bar for up and down movement, with the other end supported by the surface to be paved and moveable thereover by a pair of tandem wheels rollable over said surface and attached to said frame for pivotal movement in a verticaL plane, a taut wire attached at one end to said arm and at the other end to said bar, and switch actuator means attached to said arm and actuated by movement of said wire in response to relative movement between said arm and said bar for controlling the vertical position of the screed, said switch actuator means including a pair of spaced fingers pivotally attached at one end to said bar and straddling said wire, said fingers being coupled with a pair of switches which operate to control actuation of the screed adjusting means.
 14. The combination as defined in claim 13 wherein said fingers are attached to a housing adjacent one side of said wire and between the ends thereof, said housing being adjustable vertically relative to said bar.
 15. In combination with a slipform paving machine having a screed for spreading paving material over a surface, and selectively operable means for adjusting the height of the screed relative to the surface, a grade control apparatus having: at least one elongated, rigid bar secured to said screed and extending forwardly therefrom to overlie the surface in advance of the machine; and a sensing unit mounted on the forward end portion of said bar to travel along the surface, and comprising; a sensor mount supported on said bar; an arm pivotally supported on said forward end portion of said bar for vertical movement relative to said sensor mount, and having means thereon for traveling on said surface and moving said arm vertically in response to vertical changes in the contour of said surface; a taut member extending between said mount and arm in a normal operating position and movable vertically from said position in response to vertical movement of said arm; and means actuated by vertical movement of said member in either direction from said normal operating position, and operable to operate said selectively operable means to adjust the height of said screed.
 16. The combination as defined in claim 15 in which said means actuated by vertical movement comprise a pair of vertically spaced fingers straddling said member, a pair of actuators which control operates said selectively operable means, and coupled one with each of said fingers.
 17. The combination as defined in claim 15 in which said taut member comprises a wire stretched between said mount and said arm, and said means actuated by said vertical movement comprise switch actuators coupled with said selectively operable means and supported by said bar, said actuators having spaced operators straddling said wire to sense vertical movement thereof.
 18. In combination with a slipform paving machine having a screed for spreading paving material over a surface, and selectively operable means for adjusting the height of the screed relative to the surface, a grade control apparatus having: at least one elongated, rigid bar secured to said screed and extending forwardly therefrom to overlie the surface in advance of the machine; and a sensing unit mounted on the forward end portion of said bar to travel along the surface, and comprising: an adjustable sensor mount supported on said bar and projecting laterally thereof, said adjustable mount being moveable relative to said bar to permit said sensor unit to be offset laterally of said bar; an arm pivotally supported on said forward end portion of said bar for vertical movement relative to said sensor mount, and having means thereon for traveling on said surface and moving said arm vertically in response to vertical changes in the contour of said surface, said means for traveling including a traveler having a pair of tandem wheels; a taut member extending between said mount and arm in a normal operating position and moveable vertically from said position in response to vertical movement of said arm; and means actuated by vertical movement of said member in either direction from said normal operating position, and operable to operate said selectively operable means to adjust tHe height of said screed.
 19. The combination as defined in claim 18 in which said taut member comprises a wire stretched between said adjustable mount and said arm, and said means actuated by said vertical movement comprise switch actuators adjacent said wire and supported by said bar, said actuators being connected to said selectively operable means to control said selectively operable means to adjust the height of said screed in response to vertical displacement of said wire from said normal position.
 20. The combination as defined in claim 19 in which said arm comprises a pair of spaced longitudinally projecting rails and said sensor mount comprises a post upstanding from said adjustable mount. 